Rep. Jesse Jackson Jr., the ambitious political heir to a powerful Chicago family whose once promising future collapsed amid federal ethics investigations and a diagnosis of mental illness, resigned Wednesday from the South Side congressional seat he held for 17 years.

Jackson's downfall represents perhaps the last major political casualty in the long-running corruption scandal that sent former Gov. Rod Blagojevich to prison in March on charges he tried to sell the Senate seat of President Barack Obama.

Jackson's political star was on the rise until allegations surfaced in late 2008 that his supporters offered to raise as much as $6 million for Blagojevich in return for the governor appointing him to the Senate seat vacated by the president-elect. Though Jackson was never charged in that case, a House ethics panel investigation into his actions was ultimately eclipsed by a federal criminal probe based in Washington, D.C., into alleged misuse of campaign dollars.

Jackson's resignation letter to House Speaker John Boehner, R-Ohio, was Jackson's first acknowledgment of the ongoing federal corruption investigation.

"I am doing my best to address the situation responsibly, cooperate with the investigators, and accept responsibility for my mistakes, for they are my mistakes and mine alone," Jackson said in the two-page letter. "None of us is immune from our share of shortcomings or human frailties, and I pray that I will be remembered for what I did right."

Jackson's Washington legal team, which recently added former federal prosecutor Dan Webb, a Chicago partner at Winston & Strawn LLP, indicated that while Jackson's political fate has been settled, there's more to come in a court of law.

"We hope to negotiate a fair resolution of the matter but the process could take several months," they said in the statement.

Despite admitting "my share of mistakes," Jackson said his deteriorating health — and treatment for bipolar depression — kept him from serving as a "full-time legislator" and was the reason for his resignation.

Jackson's decision to step down came little more than two weeks after his re-election to another two-year term despite a lack of campaigning. He disappeared from the public eye in June after taking a medical leave from the House for what aides had initially described as exhaustion.

Jackson formed a political tag-team with his wife, Ald. Sandi Jackson, 7th, who over the years has received hundreds of thousands of dollars as a paid political consultant to her husband. Despite her role on the City Council, the couple maintained an upscale home in Washington and sent their children to school there. Sandi Jackson has refused to discuss her husband's political future or the investigation into his campaign spending. She could not immediately be reached for comment Wednesday.

Jackson's resignation immediately launched a field of possible successors —to be nominated and elected in special elections early next year — that could involve more than a dozen Democratic contenders, some of them political has-beens and others up-and-comers representing a new generation of leadership.

Under state law, Gov. Pat Quinn has five days to set dates for primary and general elections, which must be held by mid-March.

Some Democrats quickly offered to broker a nominee to avoid several African-American contenders splitting the vote in the heavily Democratic and majority black 2nd Congressional District, which could allow a white candidate to win. The district stretches from the South Side through the suburbs and as far as Kankakee.

Jackson's decision to leave office brought to an end a monthslong, consuming political game over the 47-year-old congressman's ability to serve his constituents.

In the congressman's public absence during the re-election campaign, both his father, civil rights leader the Rev. Jesse Jackson Sr., and Sandi Jackson sought to maintain the family's political power by offering generic statements about his health, thanking voters for their prayers and promising a return to Congress when his health permitted.

Ald. Carrie Austin, 34th, whose far South Side ward is in Jackson's district, said she wasn't surprised Jackson stepped down but was disappointed with him for misleading his constituents.

"He's lost the love and concern of the residents in his district," Austin said. "We gave him the benefit of the doubt because of his sickness, and it didn't have anything to do with that."

Jackson was first elected to Congress in 1995 in a special election to replace former Rep. Mel Reynolds, who was convicted on charges including sexual misconduct with a 16-year-old campaign aide and federal bank fraud.

In Washington, Jackson steadily moved up the ladder in a legislative chamber where seniority is a valued commodity to become Illinois' lone representative on the powerful House Appropriations Committee.

At home, he began building a local political organization in the South Side and south suburbs, an operation which successful supplanted the once powerful Shaw brothers, twins Bill and Bob, who held various posts.

A decade after entering Congress, Jackson appeared on his way to challenging then-Mayor Richard Daley. Jackson routinely criticized Daley, who had feared a strong African-American challenger. But after Democrats took control of the House, Jackson took himself out of the mayoral race to sit in the majority in Washington.

Jackson also focused on getting his wife elected to the City Council in 2007, a contest in which she defeated another of the family's political opponents, the daughter of Cook County Commissioner William Beavers.

But along the way, Jackson also found his star as one of the state's leading and most promising black politicians giving way to another South Side lawmaker, Obama, who moved from the Illinois Senate to the U.S. Senate in 2004 and won the presidency four years later.

While the congressman served as a supporter and surrogate for Obama, he also had created his share of enemies — from Daley to then-House colleague Deb Halvorson and Blagojevich. His incessant push for a south suburban airport in Peotone, which was then outside his district, launched fights with Will County officials. Blagojevich felt Jackson snubbed him by withholding a promised endorsement.

But Blagojevich found a renewed interest in Jackson after Obama's 2008 White House victory as he sought to personally and politically enrich himself through the Senate appointment process.

Jackson vigorously campaigned for the appointment publicly. But federal authorities, in making their case against Blagojevich, alleged it was Raghuveer Nayak, a Blagojevich fundraiser and longtime friend of the Jackson family, who offered to raise up to $6 million in campaign funds for the then-governor in exchange for appointing Jackson to the Senate. Jackson repeatedly said he had no knowledge of such an offer.

In June, days after Nayak was indicted on federal charges of bribing doctors to use his surgical treatment centers, Jackson's office announced that the congressman had taken a medical leave of absence from Congress two weeks earlier — a leave from which he never returned.

In subsequent months, Jackson aides refused to disclose much information about the congressman or make his doctors available for questions. He had been treated at the Mayo Clinic in Minnesota, where he was diagnosed with bipolar depression, though his whereabouts today are unknown.

Only weeks before the Nov. 6 election, reports surfaced about the federal investigation into Jackson's alleged misuse of campaign funds. Regardless of the investigation, he was a shoo-in for re-election against a little-known Republican challenger and won with 63 percent of the vote.

Now, voters in the district that includes parts of Cook, Will and Kankakee counties will be facing a new primary and general election for the seat in the weeks ahead, a proposition that Illinois State Board of Elections officials said could cost taxpayers more than $5 million. Cook County Clerk David Orr said he might seek court approval to set up the special elections on the same days as the Feb. 26 primary and April 9 general elections for municipalities in the state, a move that could save money and voter confusion.

Among some of the early names to surface as contenders are two political discards, former Cook County Board President Todd Stroger, who was dumped by county voters more than two years ago, and the man Jackson succeeded in Congress, Reynolds. Though convicted of a felony, Reynolds still has the ability to run for federal office.

Another name that surfaced was Halvorson, the former one-term congresswoman from Crete whom Jackson easily dispatched in the March primary by a margin of 71 percent to 29 percent. Others who could be in the mix, are state Sens. Toi Hutchinson, of Olympia Fields ,and Donne Trotter from the South Side as well as incoming state Sen. Napoleon Harris, a former linebacker from Flossmoor elected just weeks ago.

Among those considering a play for the seat are Alds. Anthony Beale, 9th, and Will Burns, 4th, a protege of Obama, as well as former state Reps. David Mille, of Lynwood, and Robin Kelly, of Matteson. Blagojevich's one-time attorney Sam Adam Jr. also has expressed interest in running.

Jesse Jackson Jr. was elected to Congress in 1995. Sandi Jackson was a Chicago alderman until she resigned her post in January 2013. On Feb. 15, 2013, federal prosecutors charged the couple, alleging he misused $750,000 in campaign funds and she understated their income on tax returns for six years.